Transformers are made in a wide variety of forms to meet different requirements. Design of a transformer has to consider many performance parameters, such as transforming ratio, voltage gain, magnetic leakage coefficient, loss and the like. The transformer mostly seen at present mainly includes a primary winding set, an iron core and a secondary winding set to generate magnetic coupling with the primary winding set through the iron core. In order to reduce leakage inductance of the transformer when in use, the primary winding set and the secondary winding set usually are overlapped with each other. More specifically, first, the primary winding set is wound on the iron core, then the secondary winding set is wound overlapping the primary winding set. Such an arrangement can make the secondary winding set generate as much magnetic coupling with the primary winding set as possible, thereby the leakage inductance when the transformer is in use can be reduced and the problem of electromagnetic interference (EMI) also can be decreased.
However, the requirements for the transformer at present not always demand low leakage inductance. For instance, the transformer for igniting a cold cathode fluorescent tube or an LLC resonant transformer with the leakage inductance serving as the resonant inductance requires greater leakage inductance to realize required electrical characteristics. But the aforesaid commonly adopted conventional transformers with overlapped primary winding set and secondary winding set cannot get greater leakage inductance. In addition, the conventional transformer, aside from the iron core, primary winding set and secondary winding set, often includes a coil rack for winding the primary winding set and secondary winding set, with the iron core held in the coil rack to finish assembly of the transformer. The transformer equipped with the coil rack is bulky and makes microminiaturization of the transformer difficult. Moreover, when in use power transformation also generates loss, such as copper loss, vortex loss or the like. The loss often creates waste heat. The present transformer structure does not provide heat dissipation function. With the primary winding set and secondary winding set overlapped with each other, only the primary winding set or secondary winding set at the outmost side can perform heat exchange with the external air to disperse a portion of heat. The remaining waste heat is accumulated in the primary winding set, secondary winding set or iron core, which creates undesirable effect on the transformer.